1. Field of the Invention
The present invention relates generally to technology for inspecting a wafer with a circuit using a charged-particle beam such as an electron beam and more particularly to technology for detecting foreign bodies and defects on the substrates of wafers with minute circuit patterns.
2. Description of Related Art
With the ever heightening integration of integrated circuits, the number of wiring layers of each integrated circuit is increasing and its wiring pattern is becoming more complex. At the same time, the dielectric constants of materials of insulators are being reduced and the materials of insulators are being diversified. Hoped for under the circumstances is technology for inspecting integrated circuits for electric defects speedily, stably in their manufacturing processes. It is necessary for the production of system LSI""s and the like to develop many circuit-forming processes in a short time period, which requires technology for inspecting many kinds of circuits. Available at present is technology to detect electric defects of wafers in their manufacturing processes by applying a charged-particle beam to each inspection area on the surface of a wafer and finding defects by, or based on, their distinctive contrasts in a secondary-electron image of the inspection area, the distinctive contrasts caused by the changed electric potential of the defects.
Japanese Patent Laid-Open No. 121561/1999 discloses a process for detecting discontinuity of contact holes containing transistors such as CMOS""s. According to the method, contact holes opened on n-type diffusion layers are inspected by electrifying the surface of the wafer negatively and contact holes opened on p-type diffusion layers are inspected by electrifying the surface of the wafer positively. When contact holes opened on n-type diffusion layers are inspected, the surfaces of non-defective contact holes are not electrified negatively, but the surfaces of defective ones are electrified negatively; accordingly, defective contact holes can be distinguished from non-defective ones based on their differential contrasts in a secondary-electron image of the inspection area. When contact holes opened on p-type diffusion layers are inspected, the surfaces of non-defective contact holes are not electrified positively, but the surfaces of defective ones are electrified positively; accordingly, the defective contact holes can be distinguished from the non-defective ones based on their differential contrasts in a secondary-electron image of the inspection area. Japanese Patent Laid-Open No. 87451/1999 discloses an inspection method. According to the method, while a charged-particle beam is applied to a wafer to feed it with an electric charge, a laser beam is applied to the wafer to generate carriers at p-n junctions. The currents induced by the laser beam are taken out from the substrate and measured to detect defective contacts. Thus, this method makes possible non-contact supply of currents at any spots; accordingly, it is unnecessary to form pads for electrodes on integrated circuits and integrated circuits can be inspected and analyzed with OBIC in their manufacturing processes.
Conventional methods of inspecting integrated circuits by using an electron beam have the following problems. To detect the discontinuity of a circuit pattern including contacts with various types of junctions such as CMOS""s, it is necessary to electrify the surface of each wafer positively for inspection and then electrify the surface of the wafer negatively for inspection; i.e., each wafer requires inspection twice. When the surface of a wafer shown in FIG. 2, for example, is electrified negatively for inspection, contact holes 38 opened on n-type diffusion layers 40 are electrified negatively, whereas non-defective contact holes 38 opened on the n-type diffusion layers 40 are not. When the surface of the wafer is electrified positively for inspection, defective contact holes 39 opened on p-type diffusion layers 41 are electrified positively, whereas non-defective contact holes 39 opened on the p-type diffusion layers 41 are not. In this way, each wafer, having a number of inspection areas, has to be inspected by electrifying it positively and negatively alternately. Thus, such inspection takes a considerable time, such electrification is liable to be uneven, and the sensitivity of such inspection is liable to be low. If the resistance of junctions of, for example, n-type diffusion layers of the wafer of FIG. 2 is uneven, the contact holes 38 opened on the n-type diffusion layers are electrified unevenly when the surface of the wafer is electrified positively; accordingly, some non-defective contact holes may be detected as defective based on their differential contrasts in a secondary-electron image.
In the case of the process for inspecting wafers by applying a laser beam and a charged-particle beam simultaneously and measuring the currents of the substrate, the laser beam produces electron-hole pairs, generating noises in the currents of the substrate; accordingly, it is difficult to detect differences in the faint OBIC currents. Besides, in the case of the process for measuring the currents of the substrate by applying a laser beam to generate carriers, the spatial resolution of inspection is limited by that of the laser beam applied; accordingly, it is difficult to inspect minute circuit patterns. Moreover, because dope layers are usually formed below p-n junctions under contacts, OBIC currents are influenced by the junctions between the dope layers and, hence, the sensitivity of inspection is liable to be low. Furthermore, if the circuit patterns and the Si substrate of an integrated circuit are insulated from each other by an insulator, the integrated circuit cannot be inspected by the inspection process.
Accordingly, it is an object of the present invention to provide a process for controlling the electrification of (i) the surfaces of wafers with various circuit patterns and (ii) the circuit patterns speedily, stably and inspecting the wafers for defects speedily, accurately. It is another object of the invention to provide technology for contributing toward the optimization of the manufacturing processes of integrated circuits based on data on their defects. It is yet another object of the invention to provide technology for contributing toward improving the reliability of integrated circuits by founding trouble early in their manufacturing processes and taking measures.
First of all, a process for inspecting circuit patterns including contacts with various types of junctions such as CMOS""s for defects of discontinuity will be described. According to conventional methods, each wafer with a circuit is electrified positively and negatively alternately for inspection, taking a considerable time, uneven electrification being liable to occur, the sensitivity of inspection being liable to be low. In order to solve such problems, this invention provides a means for (i) capturing a secondary-electron image of an inspection area of a wafer by applying an optical beam to the front surface of a wafer while an electron beam is being applied to the front surface and (ii) thereby reducing the influence of junctions upon the contrasts of objects being inspected in the image. With this means, a wafer can be inspected by a single inspection. An optical beam to be applied to wafers for inspection is of wavelength such that the optical beam penetrates the insulators of circuit patterns, but does not penetrate junctions of silicon. If insulators formed on circuit patterns are of SiO2 and Si3N4, an optical beam of wavelength of 200 nm or longer is used. The optical beam penetrates SiO2 and Si3N4 on the circuit patterns and is absorbed by the Si substrate to produce electron-hole pairs. Because the optical beam does not penetrates plugs on junctions, a means is provided for applying an optical beam at a slight angle from each straight line in which plugs are arranged as shown in FIG. 9 in order for the optical beam to reach the junctions.
To avoid inspecting a wafer with a circuit twice, this invention provides another means for (i) capturing a secondary-electron image of an inspection area of a wafer by applying an optical beam to the back surface of a wafer while an electron beam is being applied to the front surface of the wafer and (ii) thereby reducing the influence of junctions upon the contrasts of objects being inspected in the image. An optical beam to be used is of wavelength of 900 to 1,200 nm so that the optical beam can penetrate the Si substrate of the wafer and produce electron-hole pairs at junctions of the wafer.
Thus, by applying an ultraviolet ray or a laser beam described above to a wafer with a circuit while an electron beam is applied to the wafer, electron-hole pairs are generated at junctions of the wafer. As a result, the differences in electrification among the surfaces of plugs due to different types of junctions are eliminated; accordingly, the wafer can be inspected for defects of discontinuity by a single inspection.
According to the present invention, a wafer with a circuit is inspected for defects by detecting secondary electrons emitted from the surface of the wafer; therefore, unlike the conventional process for inspecting a wafer by detecting the currents of its substrate, a wafer with a circuit can be inspected without noises which may otherwise occur due to carriers produced by the application of a laser beam. Besides, because the spatial resolution in the inspection methods of this invention depends on the spatial resolution of secondary electrons, the methods are capable of inspecting minuter circuit patterns than the process of the prior art.
Conventional methods of inspecting wafers with an image of secondary electrons fail to detect the open contact failure of a wafer with a hole pattern formed on an insulator of Si3N4 or the like because applying a charged-particle beam to the pattern does not produce contrasts in electric potential with respect to the insulator surrounding holes. The present invention provides a means for capturing a secondary-electron image while a surface of at least one of insulators of different materials forming the circuit pattern in the inspection area is made conductive. In an example of the means, while an electron beam is applied to a wafer with a circuit, an ultraviolet ray is applied to the surface of the wafer so as to make the surface conductive. To inspect a hole pattern where the bottoms of holes are of Si3N4 and the peripheries of holes are of SiO2, an ultraviolet ray of wavelength of 150 to 200 nm is applied to the pattern so that the ultraviolet ray can penetrate the SiO2 and make the bottoms of hole conductive. The ultraviolet ray is introduced into the inspection chamber through an optical fiber or the like. In the case of a wafer with a circuit pattern of high aspect ratio, the ultraviolet ray is applied to the surface of the wafer at a small angle of incidence.
Conventional methods of inspecting a wafer with secondary-electron images of inspection areas of the wafer fail to distinguish the types of discontinuity from one another; for example, fail to distinguish open contact failure caused by a residue at the bottom of a hole from disconnection due to a void in a wiring part. The present invention provides a means for capturing a secondary-electron image by applying an optical beam to a wafer with a circuit from its front or back while an electron beam is applied to the wafer. Besides, the present invention provides a mechanism to distinguish the types of defects by comparing a secondary-electron image captured without the application of a laser beam and one captured with the application of a laser beam. When a laser beam is applied to a wafer with a circuit from its back, a laser beam of wavelength of 1.2 um or longer is used. When such a laser beam is applied to a defect, the resistance of the defect changes due to thermoelectric effect; accordingly, the degree of electrification differs from defect type to defect type, the contrasts of defects in a secondary-electron image varying among defect types. The types of defects can be distinguished from one another by comparing secondary-electron images with and without the application of a laser beam.